H2S is no joke. It doesn't take long to kill you
X2…also possible low oxygen content due to H2S and other gases outplacing the oxygen in the cistern vapor space…when something like this happens in a chemical company or refinery entering to assist is strictly prohibited without proper PPE and following proper procedure…
Condolences and prayers for the families involved…
That cistern should have been covered in a way that no one could fall in nor intentionally jump in to drink water…during drought conditions animals will jump in to drink water with no way of getting out…
It is possible the dog was hot and thirsty and jumped in to drink and cool off…and this probability led to the incredible chain of events…